1. Field of the Invention
The present invention relates to a method for coating the peripheral surface of a drum with a coating composition, and to a dipcoater for carrying out this method. In particular it relates to a dip coater for the coating of an organic photoconductor on a drum which is used in an electrophotographic apparatus for exposure to an image in order to produce an electrostatic charge pattern which may be developed by a suitable toner. The toner pattern is then transferred to a support such as plain paper, a foil for forming an offset plate, etc.
2. Description of the prior art
Organic photoconductors typically comprise one or more active layers involved in charge generation and/or charge transport with optional additional adhesion, blocking and protective layers. In one embodiment, they comprise a primer layer, a charge-generating layer and a charge-transport layer, on an electrically conductive support. The support may have the form of a drum or a belt. In the case of a drum, a cylindrical tube of aluminum or the like is clamped between two end flanges that fit on a rod that forms the rotational shaft from the drum in an electrophotographic copying or printing apparatus.
The distinct layers must be coated with a very good uniformity on the conductive support since their quality affects the quality of the final image.
The coating of a cylindrical support constitutes a particular case in the coating technology of the photographic industry, since it is a discontinuous process as distinct from the continuous coating of film and paper webs. Known methods for the coating of a drum with a coating composition are evaporation coating, spray coating, ring coating and dip coating. A good survey of these methods and their particularities is given in the article "Industrial Technology for an organic photoconductor" by Masao AIZAWA, in Denshi Shashin Gakkashi, Vol. 28, no. 2, 1989, pages 186-195.
Dip coating is mainly used for the large scale production of organic photoconductors. It is performed in practice with a coating vessel open at the top end which is filled with the coating composition, and into which a drum is dipped and then upwardly removed at a uniform speed. The thickness of the coated layer depends on the coating speed, i.e. the velocity of removal of the drum from the coating composition, and on the viscosity of the coating composition. For achieving uniformily of the coating, it is highly important that the coating speed be constant, and that the movement of the drum be free of vibrations, shocks, and other disturbances.
Know motor means such as screw spindle drives, air and hydraulic cylinders do not operate completely vibration-free, and thereby they cause problems for the satisfactory upward driving of the drum in the described application.